1. Field of the Invention
The present invention relates to a warming up control device for a catalytic converter and, more specifically, such a control device capable of determining correctly whether the catalytic converter has been warmed up.
2. Description of the Related Art
An exhaust gas purification device that utilizes a three-way reducing and oxidizing catalytic converter disposed in the exhaust gas passage of an internal combustion engine is commonly used. Generally, the catalyst used in such converters is able to purify the pollutants such as HC, CO and NO.sub.x in the exhaust gas only when the temperature of the catalyst becomes higher than a certain temperature, i.e., the catalysts in the converter do not work when the temperature of the catalyst is lower than an activating temperature.
To shorten the time required for the catalyst to reach the activating temperature after the engine starts, a warming up operation, for the catalytic converter, is usually carried out when the engine is started in cold condition. This warming up operation of the catalytic converter comprises an operation for increasing the exhaust gas temperature to heat the catalytic converter by, for example, retarding an ignition timing of the engine.
However, it is preferable to terminate the warming up operation as soon as the catalytic converter has been warmed up, since the warming up operation by retarding the ignition timing worsens the fuel consumption of the engine and also causes deterioration of the engine performance such as engine output. To terminate the warming up operation at a proper time, it is required to determine accurately whether the catalytic converter has been warmed up, i.e., whether the temperature of the catalyst has reached the activating temperature. This can be achieved by measuring the temperature of the catalyst bed of the catalytic converter using a temperature sensor disposed in the catalyst bed. However, it is not practical to dispose a temperature sensor in the catalyst bed, since it increases the cost of the catalytic converter.
To avoid this problem, various warming up control devices have been proposed to determine the completion of the warming up of the catalytic converters without using the temperature sensors disposed in the catalyst beds.
For example, Japanese Unexamined Utility Model Publication (Kokai) No. 63-26748 proposes a warming up control device for the catalytic converter which determines that the catalytic converter has been warmed up when the accumulated amount of the inlet air to the engine, after the engine starts in cold conditions, reaches a predetermined value.
The temperature of the catalyst bed increases as the total amount of the exhaust gas flows through the catalytic converter increases. The warming up control device in JPP '748 calculates the accumulated amount of the intake air of the engine (i.e., the total amount of the exhaust gas that has flowed through the catalytic converter) after the engine starts in cold conditions, and determines that the temperature of the catalyst bed has reached the activating temperature of the catalyst when the accumulated amount of the intake air reaches a predetermined value. Namely, the warming up control device in JPP '748 determines the completion of the warming up of the catalytic converter without using a temperature sensor disposed in the catalyst bed.
However, the actual temperature of the catalyst bed is determined by the amount of the heat transferred from the exhaust gas to the catalyst bed, and the amount of the heat transferred to the catalyst bed depends on the temperature of the exhaust gas as well as the amount of the exhaust gas. Therefore, the temperature of the catalyst bed also changes in accordance with the exhaust gas temperature. Since the device in JPP '748 only determines the completion of the warming up of the catalytic converter based on the accumulated amount of the inlet air (the total amount of the exhaust gas), the temperature of the catalyst bed cannot be determined correctly. Therefore, it is difficult to terminate the warming up operation of the catalytic converter at a proper time.